Elevator Trim Tab Controls

Elevator Trim Tab Controls

So your horizontal stabilizer and elevators are all covered, painted, and ready to put on the fuselage.

You need to decide how many washers go under the forward and rear mounts on the stabilizer. I did a lot of testing on 77744 and ended up with four under the front and two under the rear, so that is what I start with. The final test will be when you have zero trim deflection at cruise speed and level flight.

But back to the current task, which will be to assemble and mount the control cables that actuate the tabs. You need four cables with the fork fittings on the aft end that attach to the tab arm, and blade fittings on the other end that attach to the fork on the turnbuckle.

You need eight pulleys, four pulley guards, four washers that go between the pulleys, and four #3 bolts to attach the pulleys and guard to the ears on the stabilizer and the elevator. You need the washers between the pulleys so that there will be a close fit when the stack is placed between the ears. Then when the mounting bolt is torqued down, the center race on the pulley bearings will be captured and allow the bearing to operate properly, rather that rotate around the mounting bolt.

The two cables on each elevator start by passing over the two pulleys and into the interior of the elevator. Then to get to the attach point on the control arm of the tab, the cable has to go through a slit in the covering. This part can produce stress in the system. Not in the elevator, but in the one making the slit. I’m sure the factory had a template for the position, width, length, and angle of the slit, so it was no big deal. Here is another way if you don’t do 20 a day.

With the elevator on a bench, mount two pulleys between the ears temporarily. (you don’t have to put in the washer) and mount the control tab with the hinge pin.

Attach the fork end of one cable to the tab control arm with a pin, and position it over the top of the fabric so that it just passes over the edge of the top pulley. Put the tab in the neutral position.

See how far inside the edge of the pulley, the wire will be and match that edge distance with the cable above the pulley.

Line up the cable where it will be in the pulley.

The picture above shows the cable a little too far inside the edge of the pulley.

Now put a length of masking tape under where the cable slit will finally be. On the top surface the slit will start 9 inches from the pin on the tab control arm and be 1 inch long . Tab at neutral.

With the cable aligned on both ends, position a straight edge under the wire and sight down directly from the top . Align the straight edge just a little on the outside on one side of the wire. Draw a line, and then do the other side of the wire. This gives a box on the tape, one inch long, starting at of pin distance of 9 inches, and is just under an 1/8 inch wide.

Now reposition the wire, making sure the tab is at neutral, and take a sight picture straight down to be sure the box (slit to be) is right down the center of the wire and is the right width. Recheck the length of the box and pin distance. Do this about 20 times. Then with a new, sharp Exacto knife (#22), using a metal straight edge, slit both sides of the box. Take several passes to go through the doubler fabric and the main covering.

Slip a thin scale blade under the center on the two cuts so that you can cut the center leaving a tab on each end. This makes it easier and neater to cut a smooth end to the box.

Now tie a fish string to a piece of brass wire and attach the blade end of the cable to the fish string. Feed the wire through the slit and out the hole where the pulleys go in the front edge of the elevator.

You start from the back because the blade end of the front cable fitting can be smaller that the fork end. This minimizes the stretch of the slit in getting the end through.

If you have a wide fitting, even on the blade end, feed it into the slit from the side, pushing one side of the fabric down and the other side up.

Remove the temporary pulleys so that the cable fitting can get by, and then put them back. Now you can snug up the wire around the pulley and through the tunnel and breathe a sigh of relief that the wire sets smoothly in the new slit.

Now turn the elevator over and do the bottom slit. Since the control arm on the bottom of the tab is shorter, the cable goes through the fabric at a shallower angle. This makes the slit longer and closer to the pin. This slit starts 7 1/4 inches from the pin, and is 1 1/2 inches long. Cut the bottom slit and feed in the bottom cable, around the pulley and through the tunnel.

Now pull the two cables out in a loop, and remove the temporary pulleys. Always check to see that the cables are not crossed anywhere. Turn the elevator over with the top up and work from that side.

The task now is to reinsert the two pulleys with the washer between them. Putting some fuel lube on both sides of the washer helps a bit to hold it in place. The fit is tight so keep the drift punch in as far as possible, then remove it, ease the pulley stack into place and replace the drift punch from the bottom up. With the punch protruding about 1/8 inch through the ear, place the top hole of the guard over the punch. It is flexible enough so that you can do this even though the bottom can’t go into place yet. The guard has to be worked around the two cables one side.

Now with the correct length bolt, starting from the top against the end of the drift punch, slowly push the punch down and out as the bolt takes its place. Just as the bolt comes through the bottom ear, the bottom of the guard will slip into place over the end of the bolt into place.

Add a washer and a castle nut to the bottom, and one pulley stack is done. A point of caution! When putting the pulleys in place or removing them, keep the elevator level or front end down. If you lose your grip on one or both pulleys, they will easily slide back into the interior of the elevator and ruin your day. If they slide back a bit, do not lift up the front edge to look down into the hole to see where they went. Also do not have any tension on the cables when you remove a bolt or drift punch. The cables will pull them back where you just don’t want them to go.

If you lose a pulley back into the interior of the elevator, keep it level end to end, and raise the trailing edge to a vertical position and shake it up and down while rocking it side-to-side just a bit. You will have to remove the cables to keep the hole clear. Sooner or later, the pulleys will drop out of the hole. Now being wiser, this shouldn’t happen many more times. I would rather not go into any detail on how I know this to be true. Now put the elevator on a soft support and move it into place a couple or three inches from the stabilizer.

Feed the cables through the rear of the stabilizer and out through the rectangular holes in the ends just inside the fuselage. Keep checking to make sure that the cables are never crossed.

Now using the same method as before, insert a stack of pulleys and washer into the ears on the stabilizer. Again, insert the drift punch from the bottom. Attach the top hole of the guard onto the end of the punch, and push the punch down and out with the final mounting bolt.

Put the bottom hole of the guard in place, and put on the washer, castle nut, and torques down the nut. Add the cotter pin. Pull the cables snug and attach them to the proper turnbuckle coming off the tab control mechanism. Make sure the cables are not crossed. The other elevator is, of course, done the same way.

With the trim control in the cockpit set at neutral center position, adjust the turnbuckles until both tabs are at the neutral position and the tension in the cables is 10 pounds.

If you use a swedge-on fork on the end of the cable that attaches to the arm on the trim tab, make sure that the slot in the fork is large enough to allow the full range of pivot. Attach it temporarily with a clevis pin and be sure that it doesn’t bind. The ones I used had to be dressed open a bit to get full smooth pivoting.

The trim tab control cables in place with the minimum size slits in the fabric.

By Jack Davis|
2016-11-13T09:32:50+00:00 July 26th, 2016|Flying-Wire|Comments Off on Elevator Trim Tab Controls

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